1. Field of the Invention
The present invention relates to a brightness control circuit used in display apparatus for adjusting the brightness of display screen.
2. Description of the Prior Art
FIG. 1 is a block diagram showing a conventional brightness control circuit. In the figure, symbol 1 denotes a variable resistor which produces a brightness control voltage from a constant voltage V by being operated by the user for setting the intended brightness of a display screen. Indicated by 2 is a pre-amplifier which amplifies the original video signal in accordance with the brightness control voltage provided by the variable resistor 1. A main amplifier 3 has a constant amplification factor for amplifying the primarily amplified video signal from the pre-amplifier 2. A cathode ray tube (CRT) 4 displays the secondarily amplified video signal produced by the main amplifier 3.
Next, the operation of this prior art arrangement will be explained. The variable resistor 1 connected to a voltage source of constant voltage V divides the voltage depending on the position of its sliding electrode to produce a brightness control voltage. The brightness control voltage produced by the variable resistor 1 is fed to the brightness control voltage input terminal of the pre-amplifier 2. The pre-amplifier 2 amplifies the received original video signal at an amplification factor (several-fold) determined by the brightness control voltage, and delivers the resulting primarily-amplified video signal to the main amplifier 3.
The main amplifier 3 amplifies the received primarily-amplified video signal at the prescribed amplification factor, and delivers the resulting secondarily-amplified video signal to the CRT 4. The CRT 4 displays the secondarily-amplified video signal, with the brightness of screen being determined by the amplitude of the secondarily-amplified video signal. The conventional brightness control circuit described above is disclosed in Japanese Patent Unexamined Publication No. 63-214791, for example.
According to the conventional brightness control circuit arranged as described above, a picture displayed on the CRT 4 has its brightness determined solely by the control voltage produced by the variable resistor 1. However, as the frequency of the horizontal sync signal rises, the line scanning time of an electron beam becomes shorter, causing the screen brightness to fall because of its dependency on the electron beam intensity and exposure time length. As a result, the brightness differs among input signals of different horizontal sync frequencies unless the voltage from the variable resistor 1 is adjusted. In other words, the user is obliged to adjust the setting of the variable resistor 1 each time the input horizontal sync frequency varies.